The present invention is a novel process for preparation of epoxy resins containing triazine groups.
Epoxy resins containing triazine groups are known from Japan Kokai Tokkyo Koho No. 81 26,925 dated Mar. 16, 1981. However, the preparation of said resins involves the use of the difficult-to-obtain intermediate 2,4,6-trichloro-1,3,5-triazine. Furthermore, coupling of 2,4,6-trichloro-1,3,5-triazine with a diphenol through the chloride groups is difficult and leads to a relatively uncontrollable product mix.
The process of the present invention provides epoxy resins containing triazine groups using an easily prepared mixed cyanate of a diphenol. In the process, the polyphenol, such as 4,4'-isopropylidenediphenol (Bisphenol A) is reacted with less than a stoichiometric equivalent of a cyanogen chloride or bromide in the presence of a suitable base, such as triethylamine. This provides a mixture of monocyanate, dicyanate and, optionally, unreacted diphenol. Trimerization of this mixture provides hydroxyaromatic oligomers containing the triazine group. The oligomers and unreacted diphenol, if any, are then epoxidized using methods well known in the art.
A further benefit of the process of this invention is excellent control over the molecular weight (degree of polymerization) of the hydroxyaromatic oligomers containing triazine groups and thus the resulting molecular weight and therefore physical properties of the finished epoxy resin product. This is accomplished by varying the dicyanate (polycyanate) content of the diphenol (polyphenol) cyanate mixture used in the trimerization step of the process. A higher dicyanate content leads to higher molecular weight hydroxyaromatic oligomers through the presence of a greater number of bridged triazine groups. Conversely, a lower dicyanate content leads to lower molecular weight hydroxyaromatic oligomers.
Unreacted diphenol (polyphenol), which is preferably present, is converted to the corresponding diglycidyl ether during the epoxidation of the hydroxyaromatic oligomers. This improves overall processability of the epoxy resin. If desired, extra diphenol can be added prior to epoxidation to increase diphenol diglycidyl ether content of the finished epoxy resin product. Likewise, extra dicyanate may be added to the diphenol cyanate mixture prior to trimerization.